An air press is a mechanical device that is designed to assist in removing water from a moving web. The air press includes a positive pressure chamber, i.e. a pressure plenum placed in sealing relation with a moving substrate, and a negative pressure chamber, i.e. a vacuum device, positioned on the opposite side of the moving substrate. The moving substrate may include a tissue web sandwiched between two supporting fabrics, and the pressure differential across the moving substrate establishes airflow through the substrate. The airflow is normally used to dewater a tissue web.
The effectiveness of such an air press, as well as the effectiveness of many other types of pressure chambers, is partly a function of the seal quality that the pressure chamber forms with the moving substrate. As used herein, a "moving substrate" can refer to a paper web, a paper manufacturing felt or fabric, a roll surface, or a sandwich of a paper web between two supporting or transfer fabrics. Unfortunately, the difficulty associated with maintaining a proper seal increases, as the cross-machine length of the pressure chamber becomes longer or the pressure differential from ambient is increased. Specifically, the pressure differential between the interior of the pressure chamber and ambient conditions generates deflection forces tending to cause the pressure chamber to bow in the cross-machine direction away from the moving substrate. The bowing of the pressure chamber away from the moving substrate compromises the chamber's seal to the moving substrate. This can result in leakage either into or out of the pressure chamber or a cross-machine direction variation in load that can result in accelerated local fabric wear.
A "pressure chamber", as used herein, refers to a chamber in which the interior is at a pressure either higher or lower than atmospheric pressure.
The bowing phenomena are especially problematic on a paper machine because the pressure chamber can only be restrained from deflection on the two ends outside the manufactured web. Structural restraints positioned at locations between the ends would interfere with the paper manufacturing process, and generally are not feasible in a modern papermaking machine. Current papermaking economics dictates a paper machine as wide as possible, and what frequently limits the ability to build a wider paper machine is deflection of the cross-machine components.
Current methods for reducing the deflection of cross-machine components include increasing the cross-section of the component and hence its second moment of inertia or machining an intentional deflection into the component opposite the component's deflection when in a paper machine. A larger cross-section reduces deflection; however, this leads to an increase in the dimensions of the component that may not be practical because of limited space and greater cost. As an example, minimization of the cross-machine deflection of a paper machine roll is critical to proper tracking of fabrics and felts. Wider paper machines require roll diameters that are much larger in diameter, when compared on a proportional basis, to the increase in width between the narrower paper machine and the wider paper machine. Because the required roll diameter dramatically increases, the cost of these rolls and the space needed also dramatically increases. It is important to note that increasing the size of the components can reduce the deflection, but such an increase never eliminates the deflection.
Deflection is controlled by a material property called Young's Modulus. Young's Modulus is defined as the ratio of applied unit load, expressed as stress, to the elongation, expressed as strain, of the specimen. A higher Young's Modulus means a specimen will not deflect as much under a given load when compared to a specimen with a lower Young's Modulus. A potential method of reducing deflection could be to select a material with a higher Young's Modulus. Metals, particularly iron-bearing alloys such as steel and stainless steel, already have the highest Young's Modulus for commonly available materials. Thus, few opportunities exist for alternate material selection from which to construct paper machine components in a cost-effective manner.
Another method of reducing the effect of deflection is to manufacture the components in such a manner that the component is deflected when in the unloaded state. The component then assumes a "zero deflection state" upon application of the load. Actually, the component still is deflected due to the load, but is deflected to a desired position upon application of the load. This is accomplished by applying the expected load to the component, while it is supported at its ends, and then machining the component to the desired profile. This process is effective where the load is constant and known, but it is evident that the desired profile is only possible at the load applied during machining. If the actual load varies, or is different from the applied load during machining, the component will not have the desired profile while in use.
For all these methods, it is important to note that deflection continues to be proportional to the load or force applied to the component. For example, cross-machine deflection of a pressure chamber is proportional to the actual pressure in the chamber. Furthermore, any deflection of the cross-machine components in a paper machine is undesirable. The previous methods help to control deflection, with the aforementioned disadvantages, but the deflection of the component remains.
Therefore, what is needed is a pressure chamber that eliminates sealing problems caused by cross-machine deflection of the pressure chamber. Such a pressure chamber's cross-machine deflection will not change as the internal pressure of the chamber is changed. Relatedly, what is also lacking and needed is a more efficient method for treating a moving substrate with a fluid in a pressure chamber.